The present invention relates to data storage devices including hard disk drives, and more particularly to amplifying data signals in such devices.
A hard disk drive is one of the most widely used external storage devices for a computer. A magnetic disk or plurality of magnetic disks are typically used as a storage medium for the hard disk drive, which uses sectors as minimum data recording units. The sectors are obtained by radially dividing tracks that are obtained by concentrically dividing the disk surface. The hard disk drive has a composite magnetic head (read/write head) which comprises two devices: a read sensor and a write element. The read sensor reads data stored on the magnetic disk, whereas the write element writes data onto the magnetic disk. The magnetic head is mounted on an actuator mechanism, which is moved by a VCM (voice coil motor).
The read sensors used for reading data from the magnetic disks typically comprise magnetoresistive (MR) sensors that are used to convert and amplify the data being read from the disk into a usable format. Today's MR read sensors are resistive, where the resistance is sensitive to magnetic flux changes that represent the data stored on the disk media. Some specific types of MR sensors that are in use today include giant magnetoresistive (GMR) sensors and tunneling magnetoresistive (TMR) sensors. When a DC bias is applied to the read sensor, the read sensor's resistance change creates the read-back signal. The following equations represent the signal produced by the read sensor:Vsig=ΔR*Vbias/R Vsig=ΔR*Ibias where                Vsig=the data signal from the disk media        ΔR is the change in resistance induced by a change in the magnetic field        R=read sensor resistance        Vbias=DC voltage bias        Ibias=DC current bias.        
When a voltage bias Vbias is applied to the read sensor, the change in resistance ΔR in the magnetic field can be measured, creating the equivalent data signal Vsig. Correspondingly, for an implementation using current bias, when a current bias Ibias is applied to the read sensor, the change in resistance ΔR in the magnetic field can be measured, creating the equivalent data signal Vsig.
The signal-to-noise ratio (SNR) of the front-end system is important to processing as too much noise may cause data errors. Therefore, a required condition of both the amplifier and the bias circuit is that only a minimal amount of electronic noise be introduced into the signal by the read-back amplifier. For DC biasing, typical circuit architectures, used to remove the transducer DC bias voltage drop from the amplifier's input, involve feedback control loops and/or AC coupling capacitors. Amplifiers with a biased front-end read sensor require a means of removing the DC voltage from the read-back signal amplification. Otherwise, amplification of the read sensor DC bias would cause the amplifier to saturate and malfunction during the amplification of the read-back signal.
FIGS. 1 and 2 show two typical implementations for removing the DC bias component from the front-end of the amplifier. The implementation shown in FIG. 1 uses a feedback control loop 19 to remove the DC bias voltage. The read sensor 12 is composed of a giant magnetoresistive sensor (GMR) head 16 and the voltage source 14. The output signal 29 from the read sensor 12 is then transferred to the amplifier 18, and the outputs of the amplifier 18 are connected to both feedback loop 19 and voltage output 29. For this feedback diagram, the bias voltage 22 is interposed on the feedback loop 19, by imposing the voltage offset (Vbias 22) internal to amplifier 18. Alternatively, this voltage offset could be placed at other parts of the feedback loop such as at the read sensor 12. Internal to amplifier 18 is the bias voltage 22. The amplifier 18 output is connected as inputs to an operational transconductance amplifier (OTA) 20. The output from the OTA 20 charge or discharge capacitors 23 and 24, which provide voltages Vcc 27 and Vee 28, respectively to both the source and gate locations of transistors 25 and 26. The amplifier 18 does not amplify the front-end DC bias applied within the feedback loop and only amplifies the read-back signal.
For the feedback control scheme shown in FIG. 1, the problem is that the recovery time from switching from the write-data mode to the read-data mode can be too long for the circuit to effectively function. There are timed recovery states that can be used to change the feedback gain of the circuit, thereby attempting to decrease the recovery time. However, this method of recovery still has difficulty in reducing the overall recovery time, because changing the loop gain affects the DC operating point, which correspondingly increases the recovery time of the circuit.
The solution shown in FIG. 2 uses AC coupling capacitors 38 and 39 connecting the read sensor 35 to the read amplifier 40 to produce output signal 42. The read sensor, again, is composed of a GMR sensor 36 and signal generator 34. Here, the DC bias voltage is isolated by the AC coupling capacitors, so the input potential of the amplifier 40 is zero and the front-end DC bias is not amplified.
For the AC coupling capacitor scheme shown in FIG. 2, the recovery time from switching from write mode to read mode is too long and there is no control of the high-pass frequency response. There are switching schemes that can be implemented in a CMOS circuit to short the amplifier's input to reduce the recovery period, but these types of switching schemes can cause voltage glitches on the read sensor. Voltage glitches can destroy the read sensor by voltage overstress or punch-through, thus making the read element inoperable.
Perpendicular recording (PR) is another problem that can compound the aforementioned problems relating to recovery time within the circuit. Today's recording methods within hard disk drives utilize longitudinal recording. Longitudinal recording, as its name indicates, aligns the data bits horizontally, parallel to the surface of the disk. In contrast, perpendicular recording aligns the bits vertically, perpendicular to the surface of the disk, which allows additional room on a disk to pack more data, thus enabling higher recording densities. Due to the characteristics of the perpendicular recording read-back signal, perpendicular recording requires a lower high-pass corner frequency than longitudinal recording. This lower high-pass corner increases the recovery time when switching from the write-data mode to the read-data mode.
Despite the availability of the above-described techniques, new methods for removing the bias across magnetoresistive sensors are desirable.